Thermal Characteristics of Embedded PCMs in the Pore of Activates Carbon

Abstract

Objectives The use of phase change materials (PCMs) with the ability to absorb and release heat as an energy medium has been extensively studied where encapsulated material was used. Though these heat storage systems seem efficient but practical application suffers due to their high manufacturing cost, complexity in the manufacturing process, and eco-unfriendly nature. In this study, we developed an affordable, simpler and more efficient heat storage media by using activated carbon as a filling medium for PCM. The purpose of this study is to develop a method for caging a PCM by using the adsorption phenomenon where the PCM is adsorbed by the capillary phenomenon in the pores of activated carbon. The heat absorption and releasing characteristics of PCM embedded-activated carbon composite was investigated to comment on the applicability of the composite. Methods The PCM used in this experiment were caprylic acid (CH3(CH2)8COOH), Mn(NO3)2･4H2O, Mn(NO3)2･6H2O, Zn(NO3)2･6H2O, and n-hexadecane. The PCM embedded-activated carbon composites were prepared and studied by adopting a standard protocol. After adding and mixing the degassed activated carbon in a beaker containing a PCM, it was maintained in a high-pressure reactor of 10 atm to push the PCM into the pores of the activated carbon. The heat absorption/release capacity of the PCM embedded-activated carbon was examined within the range of 10℃ to 50℃. Results and Discussion The organic acid and the hydrated salt decreased the volume by decreasing the average distance between molecules as the temperature decreased. The heat absorption and release temperature of caprylic acid and n-Hexadecane was ~17℃ and ~15℃, respectively. Also, for Mn(NO3)2･4H2O and Zn(NO3)2･6H2O, the absorption and release of heat, i.e., latent heat was ~35℃ and ~25℃, respectively. In particular, the process of absorbing heat proceeds slowly while the process of heat release proceeds very rapidly. Moreover, Mn(NO3)2･4H2O and Zn(NO3)2･6H2O showed two inflection points in the heat release process. It was probably due to the supercooling phenomenon where the liquid and solid phase coexisted during the phase transition. From the evaluation of the mixed PCM, the inflection point of heat absorption and emission was given for the mixture of Mn(NO3)2･4H2O/Mn(NO3)2･6H2O and Mn(NO3)2･4H2O/Zn(NO3)2･6H2O. Similar trends were observed in repeated experiments. On the other hand, when Mn(NO3)S･6H2O and Zn(NO3)2･6H2O with higher water molecules content were mixed, reproducibility decreased in the subsequent heat absorption and release cycles. The heat absorption and emission characteristics of caprylic acid, Mn(NO3)2･6H2O and Zn(NO3)2･6H2O embedded activated carbon was found reproducible in multiple melting and freezing cycles and the overcooling phenomenon was overcome. The heat absorption and release characteristics of laminated PCM embedded activated carbon showed that the absorption of heat proceeded gradually from the bottom to the top, and the absorption of heat was significantly high at the top. On the contrary, heat dissipation was observed to proceed more at the bottom than at the top. The homogeneous mixing of various PCM embedded activated carbon composites resulted in a uniform distribution over a wide range of temperatures. Dozens of repeated experiments showed a similar pattern, which ensured that the activated carbon can be used as a stable thermal charging medium even with a PCM. Conclusions 1) Caprylic acid and n-hexadecane did not have the supercooling phenomena, but the hydrate-containing inorganic phase change material showed the supercooling phenomenon where both the liquid and solid phases coexisted. 2) The supercooling phenomena occurred during the cooling process that released the heat, especially when the hydrated water content was higher. On the other hand, when the phase change material is filled into the activated carbon pores, the supercooling phenomenon disappeared in the cooling process. It was considered that the external stimulus was formed because the phase change material expanded and contracted in a narrow space in the pore. 3) Mixing of hydrate-containing inorganic phase change materials which have different melting point resulted in the absorption and release of heat at the intrinsic melting point of raw material, even when filled in activated carbon pores. It was found that when the phase change materials having different melting points were used after being embedded in the activated carbon pores and mixed, they could be used as heat absorbers and release agents in a wider range of temperatures. Key words: Phase Change Material, Pore Embedded Technique, Heat Absorption and Release, Nanopore of Activated Carbon